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Identifying biological limitations of early season phenological stages of weedy rice (Oryza sativa f. spontanea Roshev.) and implications for management in California rice cropping systems


California weedy rice accessions (Oryza sativa f. spontanea Roshev.) were new pests as of 2016 with limited documentation of what influenced their phenological development. It is imperative to control weed species early in their life cycle to limit competition with crop plants, however, weedy rice is visibly undetectable before panicle development in rice fields due to the phenotypic similarities between these Oryza sativa relatives. Thus, research was needed to determine if viable weedy rice control strategies from other regions, e.g., deep flooding, would be applicable to California accessions. We hypothesized that California weedy rice accessions 1, 2, 3, 4, and 5 would behave in unique ways compared to other rice growing regions of the world due to the wet-direct seeding techniques, continuous annual flooding, and monoculture-rice agronomic practices common in California. Greenhouse, field, and laboratory studies were conducted to study early-season developmental stages of California weedy rice accessions (i.e., germination, emergence), and how these phases influence the efficacy of weed management methodologies. One management option is the stale seedbed methodology, which is dependent on maximizing seed germination of the target weed species. California weedy rice seed as well as M-206, a common rice cultivar used in California, were germinated under a range of temperature and moisture conditions. Results demonstrate that seeds are greatly influenced by moisture availability and are unlikely to germinate to the maximum extent in the presence of moisture stress. Many weedy, wild, and cultivated Oryza sativa seeds have a lower germination threshold temperature of 10°C; accession 5 seeds have a similar lower-threshold temperature, while accession 1, 2, and 3 have a lower germination threshold of 15°C. All weedy accessions have higher germination threshold temperature compared with M-206. Controlled seedling emergence studies verified our assumptions that California weedy rice accessions are likely well-adapted to flooded conditions. Weedy rice seedlings buried in the soil at either 1.3 or 2.5 cm were able to successfully emerge through the soil as well as 15 cm of flood waters. Unlike accessions from regions where drill-seeding or hand transplanting methods are utilized, California weedy rice seedlings are not suppressed with deep flooding alone. Seedlings did not emerge from the soil when buried at or below 5 cm, while seeds buried at a shallower depth of 1.3 cm resulted in maximum seedling emergence. These results can be coupled with the germination temperature and moisture parameters to maximize seedling emergence in the field. This allows for the greatest withdrawal from the soil seedbank and provides an opportunity for chemical or mechanical control. ROXY RPS® rice technology and associated chemistry, oxyfluorfen, was tested as a viable program for general weed management as well as early-season weedy rice control. Field studies demonstrated that novel ROXY RPS® rice systems will be a highly efficacious opportunity for rice-weed management. The half-life of oxyfluorfen is not well-understood in flooded soils and is only 2-3 days in puddled soils. ROXY RPS® controlled all early-emerging rice-field weeds, but there was uncertainty around whether or not this system would control weedy rice due to the observed differences in timing of emergence of accessions buried deeper (2.5 cm compared with 1.3 cm) in the soil. Additional studies with pre-plant applied oxyfluorfen were conducted in a greenhouse setting with California weedy rice accessions buried at 1.3 and 2.5 cm depths to determine if the timing of emergence based on burial depth would fall outside of the window of oxyfluorfen efficacy. There was no difference in timing of emergence of weedy rice seeds buried at different depths, and oxyfluorfen did not suppress seedling emergence the way it appeared to suppress other rice-weeds in field studies. Oxyfluorfen did cause lasting stunting and necrotic injury despite the high rate of emergence of all weedy rice accessions. These results provide information on how California weedy rice accessions are likely to behave under various environmental conditions, and what to expect when implementing various weed management methodologies.

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